Device and method for determining a magnetic field as to its intensity and direction

ABSTRACT

A device and a method for determining a magnetic field with respect to its intensity and direction at at least one detection location, and uses a first arrangement for superimposing an auxiliary magnetic field, known at least in intensity, on the magnetic field, and a second arrangement for measuring at least the direction of the magnetic field resulting from the superimposition of the magnetic field to be determined and of the auxiliary magnetic field at the detection location. The magnetic field is determined at the detection location in that the resultant magnetic field produced by the magnetic field to be determined and the auxiliary magnetic field is determined with respect to its direction for at least two different auxiliary magnetic fields; and the magnetic field to be determined is calculated therefrom. The method is especially suited for determining the intensity and direction of a magnetic field in the immediate vicinity of the surface of a magnet.

FIELD OF THE INVENTION

The present invention relates to a device and a method for determining amagnetic field, in particular with respect to its intensity anddirection in the immediate vicinity of a magnet surface.

BACKGROUND INFORMATION

Magnetic fields or magnetic field components that emerge orthogonally tomagnet surfaces can be measured using Hall probes or magnetoresistors assensors, since they are responsive to magnetic field components that areoriented perpendicularly to the sensor surface. However, the magneticfield components that run in parallel to the magnet surfaces may only beroutinely measured by these sensors at relatively large intervals of“typically” a few millimeters, since adjustment of the sensors may berequired in a direction normal to the magnet surface.

Measuring methods and measuring sensors that function on the basis ofthe Hall effect are discussed, for example, in the essay “Neuealternative Lösungen für Drehzahlsensoren im Kraftfahrzeug aufmagnetoresistiver Basis” (New Alternative Solutions for Speed Sensors ona Magnetoresistive Basis in Motor Vehicles) from VDI report no. 509,1984, VDI Publishers. Hall sensors for sensing fields directedtangentially to the magnet surface are also referred to in “Sensors andMaterials”, 5, 2 (1992), pp. 91 through 101, MYU, Tokyo, and, in theessay included therein by M. Parajape, L. Ristic and W. Allegretto“Simulation, Design and Fabrication of a Vertical Hall Device for TwoDimensional Magnetic Field Sensing”.

In addition to the sensors based on the Hall effect, so-called AMR andGMR angular-position sensors, which are based on the so-calledmagnetoresistive effect (GMR=giant magneto resistance, AMR=anisotropicmagneto resistance), can be used to measure a magnetic field componentdirected in parallel to the sensor surface.

Sensors of this kind, which are discussed, for example, in GermanPublished Patent Application No. 195 43 562 and German Published PatentApplication No. 44 08 078, may be essentially only sensitive, withintheir operative range, to the direction of the magnetic field to bemeasured and, only to a slight extent, to its intensity. At the sametime, the measured amplitude of a sensor of this kind may besubstantially temperature-dependent, which may lead to significantmeasuring errors. On the other hand, AMR or GMR angular-position sensorsmay exhibit an angular accuracy of about 0.50 (i.e., the measuringaccuracy for determining the direction of an external magnetic field tobe measured by the sensor). This angular measurement may be onlynegligibly temperature-dependent, in contrast to the intensitymeasurement.

At the same time, however, these sensors may have the advantage of alsobeing able to be used, for example, in the direct proximity of thesurface of a magnet that produces the magnetic field to be measured,i.e., typically at a distance of less than 1 mm. In this regard,reference is again made to “Neue alternative Lösungen fürDrehzahlsensoren im Kraftfahrzeug auf magnetoresistiver Basis” (NewAlternative Solutions for Speed Sensors on a Magnetoresistive Basis inMotor Vehicles) from VDI report no. 509, 1984, VDI Publishers.

The functioning of AMR and GMR angular position sensors and their usefor measuring magnetic fields is also referred to in Sensors andActuators, A21-A23, “A Thin Film Magnetoresistive Angle Detector”, 1990,pp. 795 through 798.

SUMMARY OF THE INVENTION

An exemplary embodiment and/or exemplary method in accordance with thepresent invention for determining a magnetic field is believed to havethe advantage of allowing the determination of, for the most part, anymagnetic fields whatsoever with respect to their intensity anddirection, at a selected detection location. Particularly advantageousin this context is that magnetic fields or magnetic field components,which are directed in parallel to, or emerge from, the surface, forexample, of a magnet producing these magnetic field components, can bemeasured very closely to the surface of this magnet.

Thus, as an auxiliary magnetic field required for measuring the magneticfield of interest, i.e., to be determined, one can initially use anarbitrarily produced auxiliary magnetic field, which, must be known,however, at least with respect to its intensity, preferably with respectto its intensity and direction. A Helmholtz coil pair may beadvantageously suited for producing this auxiliary magnetic field. Thecenter has a homogenous magnetic field, whose intensity and directionare known, and which can be adjusted in a defined and simple fashion,for example, by way of the coil current.

To detect the magnetic field resulting from the superimposing of theauxiliary magnetic field and the magnetic field to be measured, withrespect to its direction, an AMR or GMR angular position sensor that maybe commercially available, may be used.

In this context, a device, such as a coil having a defined and variablemagnetic field that can be produced by the coil, is integrated at thesame time in the AMR or GMR angular position sensor, to produce theauxiliary magnetic field, thereby eliminating the need for an externalcomponent, such as the mentioned Helmholtz coil pair.

Moreover, before commencing with the measurement of the magnetic fieldof interest (target magnetic field), that flows out, for example, fromthe surface of a magnet, the GMR or AMR angular position sensor used canbe utilized for calibration or test measurements. This is doneinitially, in the absence of this magnet, in that the generatedauxiliary magnet field is measured or calibrated by the GMR or AMRangular position sensor at the particular detection location, withrespect to intensity, or with respect to intensity and direction, forvarious parameter adjustments, to produce the auxiliary magnetic field.Alternatively, to measure the auxiliary magnetic field, a Hall sensorcan also be used, however. Thus, in the subsequent measurement of themagnetic field to be determined, for which then, for example, the magnetproducing the magnetic field to be determined, is placed at thedetection location in question, the auxiliary magnetic field is known ineach case for the various parameter adjustments of the auxiliarymagnetic field (for example of the coil current).

The magnetic field of interest is determined using the known auxiliarymagnetic field, by calculating the magnetic field to be measured fromthe direction, measured at the detection location, of the resultingmagnetic field produced from the superimposition of the auxiliarymagnetic field and the magnetic field to be measured.

To this end, the direction of the resulting magnetic field is determinedfor at least two auxiliary magnetic fields, at the detection locationwith respect to their various auxiliary magnetic fields, and themagnetic field to be measured is calculated therefrom with respect toits intensity and direction, employing any suitably appropriatenumerical adaptation method, which in one exemplary embodiment iscarried out or performed with the assistance of a computer program. Thisnumerical adaptation may be carried out to improve the numericalstability and the measuring accuracy using measuring data, from amultiplicity of determinations of the resulting magnetic field, given ineach case various auxiliary fields. Furthermore, with respect tomeasuring accuracy and numerical considerations, it is believed to bebeneficial for the auxiliary magnetic fields to be oriented in each caseroughly perpendicularly to the assumed direction of the magnetic fieldto be measured.

The exemplary method according to the present invention may be suitedfor determining a magnetic field emerging from a surface of a magnet, aswell as, in particular, for determining a magnetic field directedsubstantially in parallel to the surface of a magnet, in directproximity to the surface of this magnet. It may be routinely employed,for example, in the quality testing of magnets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a device for measuring a magnetic field.

FIG. 2 shows the designations and directions of the various occurringmagnetic fields.

DETAILED DESCRIPTION

FIG. 1 illustrates a magnet 13 having a surface 14, from which amagnetic field 5 to be measured (shown schematically in FIG. 1) emerges,which also exhibits, in particular, magnetic field components running inparallel to surface 14. In addition, a Helmholtz coil pair 11 isprovided, which produces an auxiliary magnetic field 6 (indicatedschematically by its direction in FIG. 1), whose intensity is adjustableby external parameters, such as a coil current. Magnet 13 is situatedroughly in the center of Helmholtz coil pair 11. Also provided is acommercially available AMR angular position sensor 12. In the exemplaryembodiment, it is an angular position sensor KMZ41 of the firm PhilipsSemiconductors, Hamburg. In place of the AMR angular-position sensor,however, a commercially available GMR angular-position sensor may beused.

AMR angular-position sensor 12 is situated at detection location 10,which, in turn, is in the immediate vicinity of surface 14 of magnet 13.The distance between detection location 10 and surface 14, inparticular, may be 0.3 mm to 3 mm. The lower proximity limit isrestricted here only by the thickness of the housing of AMRangular-position sensor 12.

At this point, reference is made to the fact that FIG. 1 is notaccording to scale, since the diameter of Helmholtz coil pair 11 issubstantially larger than the distance of surface 14 from detectionlocation 10. Thus, the differences in the intensity and direction ofauxiliary magnetic field 6 between detection location 10 and surface 14may be considered negligible.

FIG. 2 illustrates the directions of the occurring magnetic fields,which are shown as vectors and are each identified by amount, i.e.,intensity, and direction. First, auxiliary magnetic field 6, alsodenoted by H, is produced via Helmholtz coil pair 11. Magnetic field 5,also denoted by M, emerges from magnet 13. In this instance, auxiliarymagnetic field 6 may be initially oriented above the arrangement ofHelmholtz coil pair 11 such that its direction is more or less normal tothe assumed direction of magnetic field 5 to be measured.

From the superimposition of magnetic field 5 to be measured and ofauxiliary magnetic field 6, a resulting magnetic field 7 is formed atdetection location 10, the resulting magnetic field being rotated withrespect to magnetic field 5 to be measured and auxiliary magnetic field6, and also being denoted by B. The angle between the direction ofauxiliary magnetic field 6 and resulting magnetic field 7 is denoted byα. The initially unknown direction of magnetic field 5 to be measured isdenoted by angle β, which is likewise related to the direction ofauxiliary magnetic field 6.

To determine magnetic field 5, auxiliary magnetic field 6 must first beknown, with respect to its strength or with respect to its strength anddirection at detection location 10. This is done, for example, bymeasuring or calibrating the magnetic field of Helmholtz coil pair 11,before introducing magnet 13 into its field, through the use of a Hallprobe and/or a generally known GMR or AMR angular-position sensor.However, the specific method employed to determine or calibrateauxiliary magnetic field 6 in these test or calibration measurements mayinclude any suitably appropriate method. In addition, the intensity ofauxiliary magnetic field 6 must be able to be varied in defined fashion,for example by adjusting an external parameter, such as a coil currentthat generates auxiliary magnetic field 6.

The exemplary measuring method involves the fact that selectively(definably) superimposing auxiliary magnetic field 6 at detectionlocation 10 rotates the direction of resulting magnetic field 7 withrespect to the direction of magnetic field 5 to be measured and thedirection of auxiliary magnetic field 6. This rotation is dependent uponthe intensity of auxiliary magnetic field 6, but also upon the intensityof magnetic field 5 to be measured. From knowledge of the intensity orthe intensity and direction of auxiliary magnetic field 6, as well as ofthe measurement of angle of rotation a, the intensity and the directionof field 5 to be measured may be calculated or determined.

In this context, the following relation applies from the vectorsummation of the individual magnetic fields:

M cos(β) tan(α)+H tan(α)−M sin(β)=0  (1).

β is the initially unknown angle, specifying the direction of magneticfield 5 to be measured, which likewise initially has an unknownintensity M.

In equation (1), intensity H of auxiliary magnetic field 6 is known. Asdescribed, it was determined in advance in a measurement using a Hallprobe. In addition, angle α, and thus the direction of resultingmagnetic field 7 in relation to the direction of auxiliary magneticfield 6 is able to be measured using AMR angular-position sensor 12.This may be accomplished by using the sine-cosine analysis referred toin German Pubished Patent Application No. 195 43 562. To simplify thefurther procedure, it may be beneficial in this context, when, besidesthe intensity, one knows the direction of auxiliary magnetic field 6 aswell, since angle a relates specifically to this direction.

Overall, therefore, given a known auxiliary magnetic field, equation 1contains two unknowns, namely β and M. To calculate this unknown, it maysuffice to perform two measurements using two different auxiliarymagnetic fields 6, i.e., two fields which vary in intensity.

To the extent that the direction of auxiliary magnetic field 6 shouldlikewise be unknown in special cases, equation 1 also includes a thirdunknown. Accordingly, such a case requires at least three measurements,including auxiliary magnetic fields 6, which vary in intensity, but areeach constant in direction, since the angle of resulting magnetic field7 measured by AMR angular-position sensor 12 must then be based on theunknown, but constant direction of auxiliary magnetic field 6 and, asthe case may be, to the reference angle defined by it. This additionalcomplication may make it more difficult to determine β and M and, at thesame time, may lead to more serious measuring errors. For that reason,this procedure may be disadvantageous when compared to auxiliarymagnetic fields 6 which are known both with respect to intensity anddirection.

To minimize the effect of measuring uncertainties, one may perform asmany measurements as possible, or generally at least more than two orthree, including varying intensities of auxiliary magnetic field 6, inorder to obtain a multiplicity of equations (1), each with differentmeasured values for H and α. From each of these equations, one may thendetermine the values for M and β, each of these being equivalent in allthese equations, by employing a numerical adaptation (matching) method,e.g., an appropriate computer program. Therefore, one may obtainmagnetic field 5 to be measured, with respect to intensity anddirection.

The numerical stability of this adaptation method may depend on thenumber of measurements of various auxiliary magnetic fields 6 and on thesize of angle α. In the case that auxiliary magnetic field 6 andmagnetic field 5 to be measured are oriented perpendicularly to oneanother, the more closely a approaches 45° , the numerical stability ofthis adaptation method may be improved. In this case, the greatestdirectional difference may be apparent between magnetic field 5 to bemeasured and magnetic field 7 resulting from the superimposition. Forthat reason, it may be beneficial when the direction of auxiliarymagnetic field 6 and the assumed direction of magnetic field 5 to bemeasured are oriented roughly perpendicular (orthogonal) to one another.

The measuring range of the measuring method clarified here and of thecorresponding device may be approximately from 5 mT to 700 mT. In thiscontext, the lower limit is predetermined by the GMR or AMRangular-position sensor 12 used, which may require a certain minimumfield strength to at least substantially achieve its saturationmagnetization. The upper limit is based on practical considerations,since the described device may only be able to produce distinctly largerfields with difficulty. In principle, however, it should be emphasizedthat, the exemplary measuring method should not be subject to anylimitations with respect to the intensity of magnetic field 5 to bemeasured.

Comparison measurements have revealed that, at distances ofapproximately 4 mm to 12 mm to the surface of magnet 13, the intensitiesand directions of magnetic fields 5 emerging from the surface ofmagnets, determined using the described measuring method, may conformwell to comparable measurements taken using suitably appropriate Hallprobes that can likewise be employed for these distances. In this case,the measuring inaccuracy is generally less than 1 mT.

Since the specific generation of auxiliary magnetic field 6 is notsignificant, and it suffices when its intensity and direction are knownat detection location 10, the exemplary embodiment described here mayalso be implemented by additionally providing for the employed AMRangular-position sensor 12 to include a device for generating thedefined auxiliary magnetic field 6, or for AMR angular-position sensor12 and a device for generating the auxiliary magnetic field 6 to beintegrated in one component. In this case, one may do without Helmholtzcoil pair 11 and the external magnetic field produced by it. Thementioned device can be a coil that may be integrated, for example,together with AMR angular-position sensor 12 in one component.

Besides the described GMR or AMR angular-position sensors, other sensorsmay be suited for implementing the described method, provided that thedirection of resulting magnetic field 7 is measurable at detectionlocation 10.

What is claimed is:
 1. A device for use in determining at least one of adirection and an intensity of a magnetic field to be measured at adetection location, the device comprising: a first arrangement forsuperimposing an auxiliary magnetic field whose intensity is known onthe magnetic field to be measured; and a second arrangement at thedetection location for measuring the direction of a magnetic fieldresulting from superimposing the auxiliary magnetic field on themagnetic field to be measured.
 2. The device of claim 1, wherein thefirst arrangement includes a Helmholtz coil pair.
 3. The device of claim1, wherein the first arrangement and the second arrangement areintegrated in one component.
 4. The device of claim 1, wherein themagnetic field to be measured is produced by a magnet and emerges from asurface of the magnet.
 5. A device for use in determining at least oneof a direction and an intensity of a magnetic field to be measured at adetection location, the device comprising: a first arrangement forsuperimposing an auxiliary magnetic field whose intensity is known onthe magnetic field to be measured; and a second arrangement at thedetection location for measuring at least the direction of a magneticfield resulting from superimposing the auxiliary magnetic field on themagnetic field to be measured; wherein the second arrangement includesat least one of a GMR angular-position sensor and an AMRangular-position sensor.
 6. The device of claim 5, wherein the at leastone of the GMR angular-position sensor and the AMR angular-positionsensor includes a device for generating the auxiliary magnetic field. 7.A method for use in determining at least one of a direction and anintensity of a magnetic field to be measured at a detection location,the method comprising the steps of: superimposing an auxiliary magneticfield whose intensity is known on the magnetic field to be measured;determining a direction of another magnetic field resulting fromsuperimposing the auxiliary magnetic field on the magnetic field to bemeasured; and repeating the steps of superimposing and determining for adifferent auxiliary magnetic field at the detection location.
 8. Themethod of claim 7, wherein the magnetic field to be measured is producedby a magnet.
 9. The method of claim 7, wherein the magnetic field to bemeasured emerges from a surface of a magnet, and the magnetic field isdirected substantially parallel to and in direct proximity of thesurface of the magnet.
 10. A method for use in determining at least oneof a direction and an intensity of a magnetic field to be measured at adetection location, the method comprising the steps of: superimposing anauxiliary magnetic field whose intensity is known on the magnetic fieldto be measured; determining a direction of another magnetic fieldresulting from superimposing the auxiliary magnetic field on themagnetic field to be measured; and repeating the steps of superimposingand determining for a different auxiliary magnetic field at thedetection location; wherein the direction of the magnetic field to bemeasured is calculated from determined directions of the anothermagnetic field.
 11. The method of claim 10, wherein the direction of themagnetic field to be measured at the detection location is calculatedusing numerical adaptation and using intensities of the auxiliarymagnetic field and the another auxiliary magnetic field.
 12. The methodof claim 11, wherein the steps of superimposing and determining arerepeated for at least one other auxiliary magnetic field for improving anumerical stability of the numerical adaptation.
 13. A method for use indetermining at least one of a direction and an intensity of a magneticfield to be measured at a detection location, the method comprising thesteps of: superimposing an auxiliary magnetic field whose intensity isknown on the magnetic field to be measured; determining a direction ofanother magnetic field resulting from superimposing the auxiliarymagnetic field on the magnetic field to be measured; and repeating thesteps of superimposing and determining for a different auxiliarymagnetic field at the detection location; wherein determined directionsof the another magnetic field and the magnetic field to be measured aredifferent.
 14. A method for use in determining at least one of adirection and an intensity of a magnetic field to be measured at adetection location, the method comprising the steps of: superimposing anauxiliary magnetic field whose intensity is known on the magnetic fieldto be measured; determining a direction of another magnetic fieldresulting from superimposing the auxiliary magnetic field on themagnetic field to be measured; and repeating the steps of superimposingand determining for a different auxiliary magnetic field at thedetection location; wherein a Helmholtz coil pair is used for producingthe auxiliary magnetic field and the another auxiliary magnetic field.15. A method for use in determining at least one of a direction and anintensity of a magnetic field to be measured at a detection location,the method comprising the steps of: superimposing an auxiliary magneticfield whose intensity is known on the magnetic field to be measured:determining a direction of another magnetic field resulting fromsuperimposing the auxiliary magnetic field on the magnetic field to bemeasured: and repeating the steps of superimposing and determining for adifferent auxiliary magnetic field at the detection location; wherein atleast one of an AMR angular-position sensor and a GMR angular-positionsensor is used for determining the directions of the another magneticfield.
 16. The method of claim 15, wherein the auxiliary magnetic fieldand the another auxiliary magnetic field are produced by a deviceintegrated in the at least one of the GMR angular-position sensor andthe AMR angular-position sensor.
 17. A method for use in determining atleast one of a direction and an intensity of a magnetic field to bemeasured at a detection location, the method comprising the steps of:superimposing an auxiliary magnetic field whose intensity is known onthe magnetic field to be measured; determining a direction of anothermagnetic field resulting from superimposing the auxiliary magnetic fieldon the magnetic field to be measured; and repeating the steps ofsuperimposing and determining for a different auxiliary magnetic fieldat the detection location; wherein the auxiliary magnetic field and theanother auxiliary magnetic field are superimposed so that they areoriented at least approximately perpendicular to the magnetic field tobe measured.